Quantification of human basophil degranulation induced by anti-IgE, anti-IgG4, and by ionophore was performed using a flow-cytometric system. It was shown that these antibodies and ionophore can degranulate basophils in a dose-dependent manner, and that there is a wide variation in the response of basophils obtained from different individuals to these stimuli. A significant correlation was observed between the degree of degranulation induced by anti-IgE and anti-IgG4, while this was not the case for anti-IgE and ionophore. It was also shown that IgG4 myeloma protein can passively sensitize basophils. In general, degranulating efficacy was in the order of ionophore > anti-IgE >anti-IgG4, both in allergic and non-allergic individuals.